1. Field of the Invention
The present invention relates to an image forming apparatus which forms an image on a photosensitive member by an electrophotographic method and an image forming apparatus which forms an image by depositing charged ink on an electrostatic latent image bearing member electrostatically, and a method of regulating the image density in such an image forming apparatus.
2. Description of Prior Art
In electrophotographic copying machines and printers, conventionally, in order to keep a proper image density, a test pattern image with a specified area is formed on a photosensitive drum, the density of the test pattern image is optically detected, and the voltage applied to the photosensitive drum, the developing bias voltage, the toner concentration or the exposure value is adjusted according to the result of the detection. Such control is required because optimal image forming conditions to achieve a proper image density are changeable with deterioration of the photosensitive member and the developer (carrier) and with a change in environments. Especially, developers are apt to be influenced by environmental conditions, and under high temperature and high humidity, the developing efficiency declines remarkably. A decline in developing efficiency means a decline in image forming conditions, and in this case, it is necessary to heighten the voltage applied to the photosensitive drum or the toner concentration.
Now, adjustment of the voltage to be applied to the photosensitive drum and adjustment of the toner concentration are compared with each other. Adjustment of the voltage to be applied to the photosensitive drum has an advantage that the image density changes soon in response to adjustment of the voltage but has a disadvantage that underexposure occurs in the background of an image when the voltage becomes too high. Setting of a large exposure value will increase power consumption of the lamp, raise temperature and accelerate aging of the lamp and the photosensitive drum, and in order to avoid such problems, the exposure value cannot be set so large. On the other hand, adjustment of the toner concentration does not cause trouble such as underexposure. However, the toner concentration changes only gradually, and the response of the image density to adjustment of the toner concentration is slow. Moreover, when the toner concentration becomes too high, toner sticks to unnecessary portions of an image, which results in image noise.
Therefore, by combining adjustment of the voltage to be applied to the photosensitive drum with adjustment of the toner concentration, the image density can be effectively regulated.
By the way, when a large number of copies are made continuously, especially in a high-speed apparatus which makes 60 copies or more per minute, the temperature in the apparatus increases, and the characteristics of the developer degrade. Accordingly, there arise problems such as spraying of toner from the developing section and deposition of toner in the background of an image. With the above-mentioned control which is a combination of adjustment of the voltage to be applied to the photosensitive drum with adjustment of the toner concentration, although the image density is controllable to be constant, problems such as toner spraying and toner deposition occur in a multiple copying operation (especially in a duplex copying operation).